During the cardiac cycle up to 70% of the end diastolic volume of blood is ejected with each stroke. As the contraction and subsequent recoil occur, the muscular walls undergo profound changes in shape characterized by variations in wall thickness and volume-filling by shortened, thickened papillary muscles and infoldings of the walls. These effects are accompanied by large changes in ventricular stiffness, with a diastolic to systolic compliance ratio of 50 to 1. Extremely rapid filling during the initial phase of diastolic recoil then occurs without the benefit of any antagonistic muscles. This striking range of dynamic versatility is produced by the geometrical interplay of a large population of variously oriented muscle cells that undergo cyclical relative motions. The rapid and continuous relative sliding of heart muscle cells implies that they must be interconnected in a precise manner, with constraints on extreme displacements. Return to diastolic configuration is rapid and must be promoted by release of energy elastically stored during systole. Furthermore, the maintenance of patency of the myocytes throughout many contractile cycles and the efficiency of transduction of energy to the motion of cells depends critically on the viscosity, and thus on the level of hydration of the extracellular matrix. We propose to further develop and apply specific methods for visualizing and identifying connective tissue structures in heart muscle by means of antibody stains and histological stains for light microscopy and scanning, transmission, and high voltage transmission E.M. We will determine the dispositions of extracellular substances relative to myocytes, in various regions of the heart, for elastin, microfibrils, collagen types, fibronectin, and laminin. Our goals also include the determination of changes in orientation of large collagen and elastin fibers in isolated heart muscles in states of stretch and contraction relative to relaxed state in correlated structural-mechanical studies. The roles of connective tissue relative to myocytes in elastic recoil will be explored in correlated structural-kinematic studies of muscles and enzymatically isolated myocytes. Several models are proposed as working hypotheses to aid in organizing information, designing experiments, and interpreting and evaluating results.